Communications systems consist of forward and return links of communication. Forward links are generally communications from a ground station or other sender to a receiver. In satellite communications, the ground station sends a signal to a satellite, which sends the signal to a receiver. Return links are communications in the other direction, back to the ground station. Often, the forward link contains frames of data separated by contiguous unique words and containing regularly spaced pilot symbols to aid in demodulation. Pilot symbols are placed into the data stream at certain intervals or patterns to aid in the estimation of a communication signal.
Satellite and other communications systems on helicopters are typically configured with the antenna below the main rotors so that the line of slight from the antenna to the satellite or the signal broadcast is regularly blocked by the rotor blades as they rotate. These blockages adversely affect the performance of both the forward and return links.